Process for making semiconductor photo detector containing crystalline amplification layer

ABSTRACT

Described is a semiconductor photo detector comprising, between a lower electrode and an upper electrode, an optical absorption layer which generates photo carriers, receiving light and an amplification layer which amplifies the photo carriers so generated. In the semiconductor photo detector, the amplification layer is formed of a well layer which causes an avalanche phenomenon and a barrier layer which has a band gap larger than that of the optical absorption layer. The well layer is formed of a crystal substance, by which at the interface with the barrier layer, the energy value of the conduction band of the photo carriers in the well layer is lower than that in the barrier layer and at the same time, the difference in the energy value of the conduction band between the well layer and the barrier layer is larger than the band gap between the valence band and the conduction band of the well layer.

This is a Continuation-in-Part of application Ser. No. 08/739,198 filedOct. 30, 1996 now U.S. Pat. No. 5,847,418. The entire disclosure of theprior application is hereby incorporated by reference herein in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a semiconductor photo detector for use in aline image sensor for image reading of a copying machine, facsimile orthe like, or for use in a two-dimensional image sensor for image inputof a video camera or the like, particularly to a semiconductor photodetector making use of an avalanche effect for amplifyingoptically-formed carriers by impact ionization.

2. Description of the Related Art

As a device for reading light in a visible light range, CCD has beenwidely employed. A thin-film type image sensor using a semiconductorthin-film has also been proposed and it has already been industrializedin some fields. These photo detectors make use of a photo diode as alight-sensing part. They form, in principle, one or less than oneelectron for one light quantum and have no amplification effect. Ingeneral, it has been widely employed to equip a photo detector with anexternal amplification circuit, by which amplification of electrons isconducted to improve the sensitivity. According to this method, a noisecomponent in the photo detecting part is also amplified at the sametime, which inevitably leads to the lowering in the SN ratio. Such adetector is therefore accompanied with the drawback that in order toobtain a clear image using the detector, image pickup should beconducted after applying strong light to the object to be read forpreparing the conditions under which sufficient reflected light isavailable.

With a view to overcoming the above-described drawback, a detectorcapable of conducting highly-sensitive image pickup by employing asemiconductor film made of crystal Si, Se or the like and imparting thephoto-detecting part with an amplification effect has beenindustrialized in recent years. In this detector, a high electric fieldis applied to a semiconductor film made of crystal Si, Se or the like,whereby avalanche amplification (avalanche effect) is conducted. A Photodiode making use of an avalanche amplification effect (said diode willhereinafter be abbreviated as "APD") is now attracting attentions as ahighly-sensitive semiconductor photo detector which can detect feeblelight.

As this APD, there exists, as illustrated in FIG. 12(a), a singlecrystal Si pin APD comprising an n⁻ electrode 201 made of silicon towhich impurities have been doped, an SiO₂ layer 202, an n⁺ layer 203, ap layer 204 which will be an avalanche region, a p⁻ layer 205 which willbe an optical absorption region, a p⁺ substrate 206 and a p⁻ electrode207 made of silicon to which impurities have been doped.

FIG. 12(b) is a schematic view illustrating the band structure of theabove-described APD at the time when reverse bias is applied. Theincident light irradiated from the side of the n⁻ electrode 201 isabsorbed by the p⁻ layer 205 (which will be an optical absorptionlayer), whereby photoelectric transfer is conducted. Of an electron-holepair formed in the p⁻ layer 205, the electron travel toward the n⁻electrode 201 and the hole travel toward the p⁻ electrode 207,respectively. The p layer 204 (which will be a carrier multiplicationlayer) has a strong electric field so that there appears an avalanchephenomenon, that is the phenomenon forming a large number ofelectron-hole pairs by the impact ionization during traveling ofelectrons, leading to the occurrence of a multiplication effect forforming a plurality of electron-hole pairs per one photo quantum.

The multiplication factor at this time depends on the ionization rate αof electrons. The larger the α is, the higher multiplication factor canbe obtained. The term "ionization rate α" as used herein means thenumber of electron-hole pairs formed at the time when one electrontravels for a unit distance by impact ionization. The ionization rate dshows an exponential increase with an increase in the strength of theelectric field so that the larger multiplication factor can be obtainedby increasing the electric field.

The single crystal Si pin APD has been industrialized as ahighly-sensitive semiconductor photo detector which has sensitivity to arange of from visible light to near infrared light (λ=0.45-1.0 μm) andcan detect even feeble incident light. It is, however, accompanied withthe following drawbacks:

(1) it requires a high driving voltage (˜100V) because a high electricfield is applied by the externally applied voltage to cause impactionization of carriers;

(2) owing to the operation in the high electric field, leakage current(dark current) generated at the time when no light is irradiated islarge; and

(3) avalanche amplification is accompanied with the occurrence of noise(excess noise), which lowers a signal to noise ratio (SN ratio).

According to the report of R. J. McIntyre in "IEEE Transactions ElectronDevice, 13, 164(1966)", it has been elucidated that when the ionizationrate of electrons and that of holes are designated as α and β,respectively, the excess noise generated during the above-describedavalanche multiplication depends on the ratio of these ionization rates(impact ionization coefficient ratio) k=β/α, and in order to decreasethe excess noise, the ratio k may be lowered for the electronmultiplication while it may be raised for the hole multiplication, inother words, only the ionization rate of one of the carriers (electronor hole) to be multiplied may be increased.

In the case of single crystal Si, the ionization rate d of electrons ismuch larger than the ionization rate d of holes so that it is necessaryto increase only the α to decrease the excess noise. In the singlecrystal Si pin APD, however, the ionization rate α of electrons andionization rate β of holes are determined according to the electricfield strength of the avalanche region so that it is impossible tocontrol the values of α and β independently and the larger the electricfield strength, the larger the value of k. In other words, as theelectric field strength is heightened to obtain a larger multiplicationratio, the excess noise increases, inevitably leading to the reductionin an SN ratio.

The above-described report further describes that when only one of thecarriers is multiplied, the excess noise index F becomes 2. In the caseof ideal, noise-free multiplication, the index F may be 1 so that thereremains somewhat noise generating mechanism in the above case. It isconsidered as the generating mechanism that the place where ionizationoccurs fluctuates within a semiconductor photo detector so that thewhole multiplication factor fluctuates, in other words, the fluctuationbecomes a noise source. It is considered that to suppress thefluctuation and to obtain a higher SN ratio, specification of the placewhere ionization occurs within the detector is effective.

With a view to overcoming the above-described problems of the singlecrystal Si-base pin APD, APD using a super-lattice structure of anamorphous Si semiconductor is proposed in "IEEE Trans. Electron Devices,35, 1279(1988)". A description will next be made of this APD, withreference to FIGS. 13(a)-(C).

The APD using a super-lattice structure of an amorphous Si-basesemiconductor comprises, as illustrated in FIG. 13(a), a transparentelectrode 302 made of ITO, a p⁺ a--Si:H layer 303, a super lattice layer306 serving as both an optical absorption layer and a carriermultiplication layer, an n⁺ a--Si:H layer 307 and an electrode 308formed of Al, all of them being stacked one after another on a glasssubstrate 301. The super-lattice layer 306 is formed of an a--Si:H layer304 which will be a well layer and an a--SiC:H layer 305 which will be abarrier layer, said layers being stacked alternately to be 10 layers intotal. Concerning the p⁺ a--Si:H layer 303 and the transparent electrode302, and the n⁺ a--Si:H layer 307 and the electrode 308, each pair isconstructed to form an ohmic contact.

FIG. 13(b) is a schematic view illustrating the band structure of theabove-described APD at the time when no voltage is applied. In thediagram, discontinuous amounts of the energy band of the conduction bandand valence band in the hetero junction between a--Si:H and a--SiC:H areexpressed by ΔEc and ΔEv, respectively. Concerning the banddiscontinuous amount in the a--Si:H/a--SiC:H hetero junction, that ofthe conduction band is larger, and ΔEc is 0.35 eV and ΔEv is 0.10 eV.

FIG. 13(c) is a schematic view illustrating the band structure of theabove-described APD at the time when reverse bias is applied. Theincident light from the side of the p⁺ a--Si:H layer 303 is absorbed bythe super-lattice layer 306, whereby optoelectric transfer is conductedand a pair of electron and hole is formed. The electron and hole soformed travel toward the n⁺ a--Si:H layer 307 and the p⁺ a--Si:H layer303, respectively. When the electron accelerated by the electric fieldenters into the well layer 304 from the barrier layer 305 of the superlattice layer 306, its energy condition becomes higher by ΔEc, that is,the band discontinuous amount of the conduction band, which heightensthe ionization rate d of the electron in proportion. Repetition of theabove-described procedure of the electron increases the number of thecarriers.

In the case of the hole, on the other hand, no such phenomenon occursbecause the band discontinuous amount ΔEv of the valence band is small.According to the above-described APD structure, only the ionization rateα of the electron can be increased and furthermore, the place whereionization occurs can be specified at the hetero junction part so thathigh sensitivity and low excess-noise properties can be attained. Inaddition, carriers receive energy by the band offset of the heterostructure so that the electric field strength necessary for theionization of carriers can be reduced, which enables low voltage drive.

In the report by Sawada et al., in "Annual Meeting Preprint of theTelevision Society, 1995, p73", described is an APD having a gradedsuper-lattice structure in which in an a--Si:H/a--SiC:H super lattice, abarrier layer has a saw-tooth potential structure. A description willnext be made of this APD with reference to FIGS. 14(a)-(c).

The APD having a graded super-lattice structure is formed of an i-typea--Si:H 402, a graded super-lattice layer 405 serving as an opticalabsorption layer and a carrier multiplication layer, i-type a--Si:H 406,a p-type semiconductor layer 407 and a transparent electrode 408 made ofAu, all of them being stacked one after another on an n-type singlecrystal Si substrate 401. The graded super-lattice layer 405 isconstructed of an i-type a--Si:H layer 404 which will be a well layerand an i-type a--Si_(1-x) C_(x) :H (x=0-1) layer 403 which will be abarrier layer, said layers being stacked alternately to 6 layers intotal.

FIG. 14(b) is a schematic view illustrating the band structure of theabove-described APD at the time when no voltage is applied. The bandstructure of the graded super-lattice can be changed to a saw-toothstructure by continuously changing the composition ratio of thea--Si_(1-x) C_(x) :H layer within a range of x=0-1 at the time when thei-type a--Si_(1-x) C_(x) :H (x=0-1) layer 403 is deposited as a barrierlayer.

FIG. 14(c) is a schematic view illustrating the band structure of theabove-described APD at the time when reverse bias is applied. Theavalanche multiplication mechanism is fundamentally equal to that of thesuper-lattice APD illustrated in the above FIG. 13. In this diode,however, there does not exist an energy barrier against electrons at thehetero junction part in the traveling direction of electrons, whichmakes it possible to avoid cooling of electrons by which energy is lostat the time when electrons enter from the well layer 404 to the barrierlayer 403, or to prevent electrons from not being taken externally assignals owing to the accumulation of electrons in the well layer 404. Itis therefore possible to conduct a sensitivity increase and reduction innoise furthermore.

The APD which employs the super-lattice structure of an amorphousSi-base semiconductor is however accompanied with the problem that ituses an amorphous semiconductor as the carrier multiplication layer sothat electrons generated are trapped or form recombination in the film,leading to a large loss, and an amplification factor cannot beincreased.

In the super-lattice APD of an amorphous Si base semiconductor, as theband discontinuous amount ΔEc of the conduction band is about 0.34 eV,smaller than the forbidden band width Eg (1.70 eV) of the i-type a--Si:Hwhich is the well layer 404, it is necessary to apply a high electricfield to the carrier multiplication layer in order to cause an avalanchephenomenon. By this application of the high electric field,electron-hole pairs are formed from the local level in the carriermultiplication layer, leading to the problems that large dark current isformed and a high SN ratio is not available.

SUMMARY OF THE INVENTION

With the forgoing in view, the present invention has been completed. Anobject of the present invention is to provide a semiconductor photodetector having a high amplification factor at low driving voltage byusing a thin-film semiconductor which enables a high amplification ratioand low voltage drive.

With a view to attaining the above object, the amplification layer foramplifying photo carriers is constructed as follows: (i) anamplification layer is formed using a crystal substance and the meanfree path of electrons is improved substantially at the time whenelectrons transfer within the amplification layer having improved filmquality, whereby an improvement in the amplification factor is broughtabout; or (ii) the amplification layer is formed from a barrier layerand a well layer made of a crystal substance so that electrons receiveenergy at the interface therebetween, which makes it possible to improvethe mean free path of electrons substantially, thereby bringing about animprovement in the amplification factor. The film quality of theamplification layer is improved by crystallizing the amplification layerwholly or partially.

Described specifically, the present invention corresponding to the aboveconstruction (i) is characterized by that in the semiconductor photodetector having, between a pair of electrodes, at least one of saidelectrodes having a light transmitting property, an optical absorptionlayer which generates photo carriers, receiving light, and anamplification layer which amplifies the photo carriers so generated,said amplification layer is formed of a crystal substance obtained bycrystallizing an amorphous film after stacking the film.

The process for the fabrication of the above-described semiconductorphoto detector is characterized by that it is equipped with a stackingstep for stacking a pair of electrodes, an optical absorption layer andan amplification layer one after another, at least said amplificationlayer being stacked in an amorphous condition; and a crystallizing stepfor crystallizing said amplification layer in a high-temperatureatmosphere subsequent to stacking of said amplification layer in saidstacking step.

Another process for the fabrication of the above-described semiconductorphoto detector is characterized by that it is equipped with a stackingstep for stacking a pair of electrodes, an optical absorption layer andan amplification layer one after another, at least said amplificationlayer being stacked in an amorphous condition; and a crystallizing stepfor crystallizing said amplification layer by exposure to light, whichis not absorbable by said optical absorption layer but absorbable bysaid amplification layer, subsequent to stacking of said amplificationlayer in said stacking step.

In the crystallizing step for crystallizing said amplification layer byexposure to light, it is preferred to conduct melting of the interfacialregion of the optical absorption layer on the side of the amplificationlayer, together with crystallization of the amplification layer.

According to the above invention, it is possible to reduce the gap levelof the material having a large band gap, thereby improving the filmquality of the amplification layer by crystallizing the amplificationlayer formed of a semiconductor layer which amplifies electrons bymaking use of an avalanche phenomenon.

Described specifically, it is possible to substantially suppress thegeneration of electron-hole pairs in the amplification layer and tosuppress the generation of the dark current by crystallizing theamplification layer to increase the band gap between the valence bandand the conduction band and at the same time to lower the gap level.

Accordingly, it becomes possible to cause an avalanche phenomenon byapplying a high electric field to the amplification layer, to attain animprovement in the amplification factor by substantially improving themean free path of electrons to increase the electric current, and todecrease the voltage applied to the whole semiconductor photo detectorfor low voltage drive.

In addition, it becomes possible to melt the interfacial region betweenthe optical absorption layer and the amplification layer bycrystallization, thereby removing the intrastratum interfacial level,and to avoid accumulation of electrons generated by exposure to lightand suppress the disappearance of electrons at the interfacial part,thereby bringing about an improvement in the amplification factor.

The present invention corresponding to the above-described (ii) has, ina semiconductor photo detector having, between a pair of electrodes, atleast one of said electrodes having a light transmitting property, anoptical absorption layer generating photo carriers, receiving light andan amplification layer which amplifies the photo carriers so generated,the following structure.

The amplification layer has a barrier layer having a band gap largerthan that of the optical absorption layer and a well layer stackedcontiguously on said barrier layer.

The well layer is formed of a crystal substance by which, at theinterface with the barrier layer, the energy value of the conductionband of the photo carriers in the well layer is lower than that in thebarrier layer and at the same time, the difference in the energy valueof the conduction band of the photo carriers between the well layer andthe barrier layer is larger than the band gap between the valence bandand the conduction band of the well layer.

The process for the fabrication of the above-described semiconductorphoto detector is characterized by that it is equipped with a stackingstep for stacking a pair of electrodes, an optical absorption layer, abarrier layer and a well layer one after another, at least said welllayer being stacked in an amorphous condition; and a crystallizing stepfor crystallizing said well layer in a high-temperature atmospheresubsequent to stacking of at least said well layer in the stacking step.

In the crystallizing step, the barrier layer and the well layer may becrystallized after stacking at least the barrier layer and well layer inan amorphous condition and then after the stacking step, exposing themto the light of a wavelength which is absorbable thereby.

Alternatively in the crystallizing step, the well layer may becrystallized after stacking at least the barrier layer and well layer inan amorphous condition and then after the stacking step, exposing themto light which is not absorbable by the barrier layer but is absorbableby the well layer.

According to the above-described invention, the well layer of theamplification layer which is formed of a semiconductor layer whichamplifies electrons by making use of an avalanche phenomenon is formedof a crystal substance, whereby the gap level can be reduced and a highamplification factor can be attained even at low driving voltage and atlow dark current.

Described specifically, the well layer is formed of a crystal substancehaving a small band gap so that at the interface between the well layerand the barrier layer, the energy value of the conduction band of thephoto carriers in the well layer can be made lower than that in thebarrier layer and the difference in the energy value of the conductionband of the photo carriers between the well layer and the barrier layercan be made larger than the band gap between the valence band and theconduction band of the well layer.

Electrons which are traveling from the barrier layer to the well layerreceive, at the interfacial part therebetween, the energy correspondingto the difference of the conduction band and cause an avalanchephenomenon in the well layer. This energy which electrons receive at theinterfacial part is made larger than the band gap between the valenceband and the conduction band of the well layer so that the avalanchephenomenon can be caused only by this energy and low voltage drive canbe conducted.

In other words, the film quality of the semiconductor thin film in theamplification layer composed of the barrier layer and the well layer canbe improved substantially, an improvement in the amplification factorcan be attained by substantially improving the mean free path ofelectrons, thereby increasing the electric current; and generation ofelectron-hole pairs from the semiconductor thin film can besubstantially suppressed by low voltage drive.

The advantages of the present invention can be summarized as follows:

According to the present invention, the film quality of thesemiconductor thin film which is an amplification layer can be improvedlargely, an improvement in the amplification factor can be attained byan increase in the electric current brought by a substantial improvementin the mean free path of electrons and generation of electron-hole pairsfrom the film can be suppressed substantially.

It therefore becomes possible to fabricate a semiconductor photodetector which enables a high amplification factor even at low appliedvoltage.

Accordingly, compared with an amplification means using a conventionalexternal amplification circuit, it is possible to amplify signal chargeswith remarkably low noise and to attain size reduction and costreduction of, for example, a high-speed and high-resolution image inputsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a schematic cross-sectional view illustrating the structureof the semiconductor photo detector according to the present invention,(b) is an energy band diagram illustrating the band structure of theinterfacial part between the optical absorption layer and theamplification layer of the semiconductor photo detector, and (c) is anenergy band diagram illustrating the band structure of the semiconductorphoto detector at the time when reverse bias is applied;

FIG. 2 is a schematic view illustrating the C concentration in theinterface region between the optical absorption layer and theamplification layer of the semiconductor photo detector;

FIG. 3 is a schematic view illustrating the formation method of aninterface region between the optical absorption layer and theamplification layer of the semiconductor photo detector;

FIG. 4 is a voltage-gain characteristic diagram of the semiconductorphoto detector according to this embodiment;

FIG. 5 is a schematic cross-sectional view illustrating the structure ofthe semiconductor photo detector according to another embodiment;

FIG. 6 is an energy band diagram illustrating the band structure of thesemiconductor photo detector according to another embodiment, beforeexposure to laser;

FIG. 7 is a voltage-gain characteristic diagram of the semiconductorphoto detector according to another embodiment;

FIG. 8(a) is a schematic cross-sectional view illustrating the structureof the semiconductor photo detector according to the present invention,(b) is an energy band diagram illustrating the band structure of thesemiconductor photo detector at the time when no voltage is applied, and(c) is an energy band diagram illustrating the band structure of thesemiconductor photo detector at the time when reverse bias is applied;

FIG. 9 is an energy band diagram illustrating the band structure, at thetime when no voltage is applied, of the semiconductor photo detector inwhich the well layer and the barrier layer have been crystallized;

FIG. 10 is a schematic cross-sectional view illustrating the structureof the semiconductor photo detector according to another embodiment;

FIG. 11(a) is a schematic cross-sectional view illustrating thestructure of the super-lattice employing semiconductor photo detectoraccording to another embodiment, and (b) is an energy band diagramillustrating the band structure of this semiconductor photo detector atthe time when no voltage is applied;

FIG. 12(a) is a schematic cross-sectional view illustrating thestructure of a conventional single crystal Si APD, (b) is an energy banddiagram illustrating the band structure of this single crystal Si APD atthe time when no voltage is applied, and is an energy band diagramillustrating the band structure of this single crystal Si APD at thetime when reverse bias is applied;

FIG. 13(a) is a schematic cross-sectional view illustrating thestructure of a conventional amorphous Si-base super-lattice APD, (b) isan energy band diagram illustrating the band structure of this amorphousSi-base super-lattice APD at the time when no voltage is applied, and(c) is an energy band diagram illustrating the band structure of thisamorphous Si-base super-lattice at the time when reverse bias isapplied; and

FIG. 14(a) is a schematic cross-sectional view illustrating thestructure of a conventional amorphous Si-base graded super-lattice APD,(b) is an energy band diagram illustrating the band structure of thisamorphous Si-base graded super-lattice APD at the time when no voltageis applied, and (c) is an energy band diagram illustrating the bandstructure of this amorphous Si-base graded super-lattice at the timewhen reverse bias is applied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will next be made of one embodiment [corresponding to theconstruction described in the above (i)] of the semiconductor photodetector relating to this invention, with reference to its structure andband structure in FIGS. 1(a)-(c). FIG. 1(a) is a cross-sectional viewillustrating the structure of the semiconductor photo detector accordingto the present invention, FIG. 1(b) is a schematic view illustrating theband structure at the interfacial part between an optical absorptionlayer and an amplification layer of the semiconductor photo detector,and FIG. 1(c) is a schematic view illustrating the band structure of thesemiconductor photo detector at the time when reverse bias is applied.

The semiconductor photo detector according to the present invention isformed by stacking a lower electrode 12, an optical absorption layer 13,an amplification layer 14 and an upper electrode 15 one after another onan insulating substrate 11. The optical absorption layer 13 is a layerforming photo carriers, receiving light. The photo carriers generated bythe optical absorption layer 13 are amplified by the amplification layer14. The amplification layer 14 is formed of a crystal substance obtainedby crystallizing an amorphous film after stacking it. In thisembodiment, light is irradiated downwards so that the upper electrode 15is formed of a light transmitting material.

The amplification layer 14 is crystallized by exposure to laser lightfrom an excimer laser or the like or crystallized in a high-temperatureatmosphere such as infrared-ray lamp annealing or the like. By thecrystallization, the gap level (local level density) of theamplification layer 14 which amplifies electrons by making use of anavalanche effect can be reduced to the level free from practicalproblems even if a material having a large band gap is selected for theamplification layer.

By crystallizing the amplification layer 14, the interfacial partbetween the amplification layer 14 and the optical layer 13 is moltenduring the crystallizing step of the amplification layer 14 and atomsfrom the amplification layer 14 and the optical absorption layer 13diffuse properly in the interfacial region and form an interface so thata change in the band becomes continuous [FIG. 1(b)]. By melting inaddition to crystallization, a definite surface region disappears,whereby the surface level which otherwise becomes a recombination centersubstantially disappears. Accordingly, the band structure at the timewhen reverse bias voltage is applied shows a downward monotonousinclination as shown in FIG. 1(c) so that when the photo carriers formedby the optical absorption layer 13 travel, they can travel smoothlywithout loss of electrons caused by the energy barrier, wherebyaccumulation, at the interfacial part, of electrons generated byexposure to light can be avoided, disappearance of electrons at theinterfacial part, which has so far been a problem, can be suppressed anda substantial improvement in the amplification factor can be attained.

In the above construction, a material having a large band gap can beemployed as the amplification layer 14 so that the generation of darkcurrent, at the time when reverse bias voltage is applied, caused by theelectric field other than the avalanche amplification can be suppressed.In addition, by the crystallization of the amplification layer 14, thegap level (local level density) at the time when reverse bias voltage isapplied can be reduced to a level free from the practical problems andthe formation of electrons which will be a cause for the dark currentcan be suppressed. As a result, a high amplification factor can beobtained at low driving voltage and low dark current in thesemiconductor photo detector.

A description will next be made of another embodiment [corresponding tothe construction described in the above (ii)] of the semiconductor photodetector relating to this invention, with reference to the structure andband structures in FIGS. 8(a)-(c). FIG. 8(a) is a cross-sectional viewillustrating the structure of the semiconductor photo detector accordingto the present invention, FIG. 8(b) is a schematic view illustrating theband structure of the semiconductor photo detector at the time when novoltage is applied, and FIG. 8(c) is a schematic view illustrating theband structure of the semiconductor photo detector at the time whenreverse bias is applied.

The semiconductor photo detector according to this invention is formedby stacking a lower electrode 2, an amplification layer 3, an opticalabsorption layer 4 and a upper electrode 5 one after another on aninsulating substrate 1. The optical absorption layer 4 is a layergenerating photo carriers, receiving light. The photo carriers sogenerated by the optical absorption layer 4 are amplified by theamplification layer 3. The amplification layer 3 is formed of a welllayer 3a positioned on the side of the electrode 2 and the barrier layer3b positioned on the side of the optical absorption layer 4. In thisembodiment, the upper electrode 5 is formed of a light transmittingmaterial because light is irradiated downwards.

The barrier layer 3b is, as illustrated in FIG. 8(b), has a band gaplarger than that of the optical absorption layer 4. The barrier layer isconstituted so that by continuously changing the composition ratio ofthe material forming the barrier layer 3b, the band gap shows anincrease from a value equal to that of the optical absorption layer 4over a range from the side of the optical absorption layer 4 toward theside of the well layer 3a, and reaches the maximum value at theinterface with the well layer 3a. In addition, when drive voltage isapplied between the lower electrode 2 to the upper electrode 5 (whenreverse bias is applied), the energy value of the conduction band of thephoto carriers in the barrier layer 3b shows, as illustrated in FIG.8(c), a flat change or monotonous decrease toward the travelingdirection of the photo carriers. This is because the photo carriersformed by the optical absorption layer 4 can travel in the barrier layer3b smoothly without a loss of electrons by the energy barrier at thetime when reverse bias is applied.

The structure of the present invention is characterized by that the welllayer 3a of the amplification layer 3 is formed of a crystal substancesuch as substance, for example, poly-Si film, single crystal substanceor micro-crystal substance. The well layer 3a formed of a crystalsubstance can be prepared by (i) stacking the material of a crystalsubstance as the well layer 3a, (ii) crystallizing an amorphous materialin a high-temperature atmosphere after stacking it, (iii) crystallizingan amorphous material by exposure to laser after stacking it, or thelike. As the above method (ii), the well layer 3a can be deposited,followed by crystallization or the well layer 3a and the barrier layer3b can be deposited continuously, followed by crystallization of bothlayers. As the method (iii), the well layer 3a and the barrier layer 3bcan be deposited continuously, followed by exposure to laser light ofdifferent wavelengths to crystallize only the well layer 3a or both thewell layer 3a and the barrier 3b can be crystallized. Specific examplesof the crystallization means for (ii) include infrared ray lampannealing and furnace annealing, while those for (iii) include excimerlaser and blue laser.

By forming the well layer 3a from a crystal substance having a smallband gap, the energy value of the conduction band of the photo carriersin the well layer 3a at the interface with the barrier layer 3b can beset lower than that in the barrier layer 3b and at the same time, thedifference Δc in the energy value of the conduction band of the photocarriers between the well layer 3a and the barrier layer 3b can be setlarger than the band gap (forbidden band width Eg) between the valenceband and the conduction band of the well layer 3a.

According to this structure, the electrons which are traveling from thebarrier layer 3b to the well layer 3a receive, at the interfacial parttherebetween, the energy corresponding to the difference of theconduction band (potential energy is converted to the kinetic energy)and by the impact ionization to the lattice in the well layer 3a, theyform electron-hole pairs and cause an avalanche phenomenon. At thistime, the energy (ΔEc) which electrons receive at the interface islarger than the band gap (forbidden band width Eg) between the valenceband and the conduction band of the well layer 3a so that only thisenergy is enough for causing the avalanche phenomenon.

According to the semiconductor photo detector having the above-describedstructure, the well layer 3a which amplifies electrons by making use ofan avalanche effect is formed of a crystal substance having a small bandgap so that it becomes possible to cause the avalanche phenomenon usingonly the energy (ΔEc) which electrons receive at the interface. Novoltage application is required for the avalanche phenomenon so that thelow drive voltage can be attained. When the reverse bias voltage isapplied, the gap level can be reduced to the level free from practicalproblems, whereby the formation of electrons which become a cause forthe dark current can be suppressed. As a result, in this semiconductorphoto detector, a high amplification factor can be obtained at lowdriving voltage and low dark current.

Furthermore, by stacking the well layer 3a and the barrier layer 3b eachformed of an amorphous material and crystallizing both layers afterstacking, the interfacial part therebetween is molten in thecrystallizing step of the well layer 3a and electrons from the welllayer 3a and barrier layer 3b diffuse properly in the interface regionand form an interface, whereby a change in the energy band becomescontinuous as shown in FIG. 2 at the interface. By melting in additionto crystallization, the definite interfacial region is lost, whereby thesurface level which becomes a recombination center almost disappears.

It is therefore possible to suppress the disappearance of electrons atthe interfacial part and to attain a substantial improvement in theamplification factor.

In the method where the well layer 3a and the barrier layer 3b eachformed of an amorphous material are stacked one after another and thenonly the well layer 3a is exposed to laser for crystallization, somewhatdiffusion appears at the interfacial part between the well layer 3a andthe barrier layer 3b by exposure to light for melting. Even this methodis more effective for suppressing the disappearance of electrons at theinterfacial part than the method [described in (i)] where the materialof a crystal substance is stacked as the well layer 3a.

A description will next be made of specific embodiments of thesemiconductor photo detector of the present invention corresponding tothe above-described constitution (i).

Embodiment 1

The semiconductor photo detector according to this embodiment isfabricated by stacking a lower electrode 12, an optical absorption layer13, an amplification layer 14 and an upper electrode 15 one afteranother on an insulating substrate 11.

Described specifically, the lower electrode 12 is formed by conductingdesired patterning using a metal such as Cr, Al, Ta, Ti, Mo or Ni on theinsulating substrate 11 made of glass, ceramic or the like. As themetal, alloys of the above-exemplified metal may be used. Othermaterials having conductivity can also be used. On the lower electrode12, a--Si is deposited, as a semiconductor layer of the opticalabsorption layer 13, to the film thickness of 100-1000 nm by the plasmaCVD method.

General conditions for depositing the above film are as follows:

Gas used and flow rate: SiH₄ (100%) 100 sccm

Pressure: 0.3 Torr

Substrate temperature: 250° C.

RF Power: 50 W

Examples of the film deposition method include, in addition to the aboveplasma CVD method, ECR method and optical CVD method. The sputteringmethod or vapor deposition method can also be employed.

As the optical absorption material of the optical absorption layer 13,a--Si is employed, however, it is possible to add a suitable additive toSi depending on the wavelength of the light to be detected. Forattaining an improvement in the sensitivity to the shorter wavelengthregion, a desired amount of C, N, O or the like is added, while for theimprovement of the sensitivity to the longer wavelength region, anelement such as Ge may be added.

As a base material for the optical absorption layer 13, it is desired touse Si to detect the visible light range. It is also possible to useother thin-film materials such as Ge, Se, CdS, CdSe or PbS. When such amaterial is used, the material and preparation process, and alsocrystallization means should be selected in full consideration of thethermal influence which will be exerted in a subsequent crystallizingstep.

Continuously on the optical absorption layer 13, an avalancheamplification layer 14 is formed using the formation method similar tothat employed for the optical absorption layer. As the amplificationlayer 14, used is a--SiC having a larger band gap than that of theconventionally used material. It is needless to say that as theamplification layer 14, a material having a high electron formationefficiency is suited. Furthermore, it is necessary to select a materialhaving a high α/β ratio (an electron multiplication factor/holemultiplication factor ratio) should be selected to suppress thegeneration of the excess noise at the time of amplification.

Furthermore, it is also necessary to select a material which hasintra-film gap level as small as possible to avoid the formation ofelectrons from the gap level of hole-electron pairs at the time when ahigh electric field is applied, which is a principal cause for the darkcurrent.

In this embodiment, a--SiC is employed as the amplification layer 14 butit is also possible to use Se, a--SiN, a--SiO, CdS and ZnS and compoundsthereof. Upon selection, it should be noted that as incident light isallowed to enter from the side of the amplification layer 14, theoptical absorption coefficient of the amplification layer 14 is madesmaller enough compared with that of the optical absorption layer 13.

General conditions for forming a--SiC are as follows:

Gas used and flow rate: SiH₄ 100 sccm, C₂ H₆ 10 sccm

Pressure: 0.5 Torr

Substrate temperature: 250° C.

RF Power: 50 W

Film thickness: 10-1000 nm

At the time of formation, the C content in the amplification layer 14 isadjusted by changing the flow rate of C₂ H₆ gas as needed. The C/Siratio is usable within a range of from 0.1 to 3 and it is selectedaccording to the wavelength of the light to be detected. When a visiblelight range is detected, the ratio falling within a range of from 0.5 to2 is preferred, while the band gap energy is preferably 1.5-3.5 eV.

Then, a--SiC which forms the amplification layer 14 is crystallizedusing an excimer laser. In addition to this, Ar laser, CO₂ laser,infrared flash annealing can be employed for the formation. It shouldhowever been borne in mind that the crystallization method is selectedso that the heat rays are absorbed sufficiently in the amplificationlayer 14 to crystallize the layer, and they are absorbed only by theamplification layer 14 and do not exert influence on the opticalabsorption layer 13 below the amplification layer 14.

In this embodiment, the optical absorption layer 13 (a--Si layer) existsbelow the amplification layer. If heat rays reach the optical absorptionlayer and cause crystallization of a--Si, this layer cannot exhibitdesired properties as the optical absorption layer 13. In the case ofthe excimer laser, if the film thickness of a--SiC and laser strengthare set properly, it is possible to crystallize only the amplificationlayer 14 and maintain the optical absorption layer 13 (a--Si layer)without crystallization, because its wavelength exists within theultraviolet region and even a--SiC having a large band gap has asufficiently large absorption coefficient.

Typical examples of the crystallizing conditions are shown below:

Laser used: KrF

Laser strength: 50-500 mW/cm²

Pulse width: 50 nsec

Pulse irradiation frequency: 1-50 times

Under the above conditions, crystallization for the amplification layer14 (a--SiC layer) to the thickness of about 200 nm was conducted.

The gap level (local level density) of a--SiC decreased largely from10¹⁸ /cm³, the level before crystallization, to 10¹⁶ /cm³ as the resultof the crystallization under the above conditions. In order to cope withthe crystallization of the amplification layer 14 thicker than theabove, a laser of a longer wavelength can be used or laser strength canbe increased. It should be noted at this time that as shown in FIG. 3,when the amplification layer 14 is exposed to laser irradiateddownwards, the amplification layer 14 is crystallized over the wholethickness direction and at the same time, the optical absorption layer13 (a--Si layer), which is below the amplification layer 14, is slightlymolten. By this crystallization and slight melting, C diffuses from theamplification layer 14 (a--SiC layer) and mixes in the upper layerportion of the optical absorption layer 13 (a--Si layer), therebylowering the C concentration in the interfacial part from theamplification layer 14 toward the optical absorption layer 13, wherebythe continuous content distribution of C can be obtained.

Accordingly, as shown in FIG. 1(b), even if there exists an energy gap,as shown by a dotted line, between the optical absorption layer 13 andthe amplification layer 14 at the time of stacking, the band gap becomessmall in the boundary region layer on the side of the optical absorptionlayer 13 owing to the release of H (hydrogen) from the opticalabsorption layer 13, while the band gap becomes large in the boundaryregion layer on the side of the amplification layer 14 owing to thediffusion of C, by which the energy bands on the interface between theoptical absorption layer 13 and the amplification layer 14 can be madecontinuous and accumulation of electrons at this part can be avoided.Needless to say, no definite interface is formed so that an interfaciallevel which will cause a trouble is not formed at all.

In the boundary layer on the side of the optical absorption layer 13,the band gap becomes smaller owing to the release of H (hydrogen) fromthe optical absorption layer 13 as described above, whereby aconstriction is formed. This constriction has effects for acceleratingthe transfer of electrons from the optical absorption layer 13 to theamplification layer 14, thereby contributing the improvement in theamplification factor.

After crystallization, it is possible to conduct various treatments, asneeded, to a--SiC which is the amplification layer 14, with a view toimproving its film quality. Examples of the effective treatment for usein this invention include hydrogenation treatment by the plasma CVD orECR CVD apparatus and annealing in the forming gas (H₂, N₂). When ana--SiC layer is treated, H contained in the film is released during thecrystallizing step so that in order to make up for it and to reduce theintra-film gap level, it is effective to conduct hydrogenation treatmentby plasma.

As the final step, ITO or SnO₂ is deposited to form a transparentelectrode, followed by patterning, whereby the upper electrode 15 isformed. Any material can be used for the electrode insofar as it hasboth a sufficient light transmitting property and small resistance.Usable examples include ultra-thin films of metals such as Al, Cr, Ta,Ti and Ni and alloys thereof, and also suicides thereof.

In this embodiment, incident light is irradiated from the side of theupper electrode 15 existing in the upper part of the device. It is alsopossible to irradiate incident light from the lower part of the device,using the insulating substrate 11 as a light transmitting material. Atthis time, the upper electrode 15 is not required to be transparent.

When reverse bias voltage is applied to the above-describedsemiconductor photo detector, the electric field is applied to theamplification layer 14, thereby causing avalanche amplification. Theband structure at this time shows a downward monotonous inclination sothat accumulation, at the interfacial part, of electrons generated bythe optical irradiation can be avoided, disappearance of electrons atthe interfacial part can be suppressed and a drastic increase in theamplification factor can be attained. Furthermore, the amplificationlayer 14 (a--SiC) has higher resistance than the optical absorptionlayer 13 (a--Si) has so that most of the voltage to be applied to thesemiconductor photo detector is applied to the amplification layer 14,whereby the electric field of the amplification layer 14 can beheightened while the driving voltage of the semiconductor photo detectoris lowered.

As the amplification layer 14, crystallized a--SiC is used so that theband gap of the amplification layer is enlarged and dark current formedby the electric field at the time when reverse bias voltage is appliedcan be suppressed. By crystallization, the gap level (local leveldensity) can be reduced to 10¹⁶ /cm³ which is necessary for suppressingthe dark current at the time when reverse bias voltage is applied.

Concerning the semiconductor photo detector fabricated by theabove-described method, one example of the characteristics in the casewhere the optical absorption layer 13 (a--Si) and the amplificationlayer 14 (a--SiC) are formed to give a film thickness of 500 nm and 200nm, respectively is shown in FIG. 4.

As is apparent from this, the photo detector according to thisembodiment has a gain of about 50 at the applied voltage of 10V, whichshows an improvement in the amplification factor by one figure or morecompared with the conventional one. It is of course presumed that afurther improvement can attained by increasing the film thickness andapplied voltage.

The dark current at the applied voltage of 20 V is 1 nA/cm² or lower,from which it is confirmed that the dark current can be lowered by twofigures or more.

Embodiment 2

A description will next be made of the second embodiment, with referenceto FIGS. 5 and 6. In this embodiment, the amplification layer has alaminated structure of films different in the band gap (forbidden bandwidth).

Described specifically, on a insulating substrate 21 made of glass,ceramic or the like, a lower electrode 22 is formed by patterning usinga metal such as Cr, Al, Ta, Ti, Mo or Ni into a desired shape. Alloys ofthe above-exemplified metal can also be used. It is needless to say thatother materials can also be used.

On the lower electrode 22, an electron-injection inhibition layer 23 isformed by the deposition of a p⁺ a--Si layer to a film thickness of 50nm. Alternatively, it is possible to insert an insulating film as thinas 1-50 nm as the electron-injection inhibition layer. As the insulatingfilm, generally employed SiO_(x), Si_(x) N_(y) or the like can be used.

On the electron-injection inhibition layer 23, formed is an opticalabsorption layer 24 made of a semiconductor layer obtained by depositinga--Si to the film thickness of 100-1000 nm by the plasma CVD method.General conditions for the deposition of the optical absorption layer 24are as follows:

Gas employed and flow rate: SiH4 (100%) 100 sccm

Pressure: 0.3 Torr

Substrate temperature: 250° C.

RF Power: 50 W

Examples of the method usable for film deposition include, in additionto the plasma method, ECR method and optical CVD method. The sputteringmethod or vapor deposition method can also be employed.

As the optical absorption material, a--Si is employed but it is alsopossible to add a suitable additive to Si depending on the wavelength ofthe light to be detected. For the improvement in the sensitivity to theshorter wavelength region, a desired amount of C, N, O or the like canbe added, while for the improvement in the sensitivity to the longerwavelength region, it is possible to add an element such as Ge.

As a base material for the optical absorption layer 24, it is desired touse Si to detect the visible light range. It is also possible to useother thin-film materials such as Ge, Se, CdS, CdSe or PbS. When suchmaterials are used, however, the material and formation method, and alsocrystallization means should be selected in full consideration of thethermal influence which will be exerted on the subsequent crystallizingstep.

Continuously on the optical absorption layer 24, an amplification layer25 is formed using the formation method similar to that employed for theoptical absorption layer. This embodiment is characterized by that theamplification layer 25 is formed of an avalanche layer 25a made of acrystal substance and a barrier layer 25b, and each amplification layeris formed of a laminated layer obtained by stacking two a--SiC layersdifferent in the band gap (forbidden band width) in plural number oftimes.

The reason why the amplification layer 25 is formed of a laminated layeris because in the crystallized avalanche layer 25a, an avalancheamplification effect is caused as in the amplification layer 14according to the first embodiment and at the same time, avalanchemultiplication is caused at the interfacial part by the energycorresponding to ΔEc between the avalanche layer 25a and the barrierlayer 25b.

As the amplification layer 25, a material which has high electronformation efficiency is suited as described in the first embodiment. Inaddition, it is necessary to select a material having a high α/β ratio(an electron amplification factor/hole amplification factor ratio) tosuppress the generation of excess noise at the time of amplification. Itis also necessary to select a material having a small intra-film gaplevel to avoid the formation of electrons from the gap level ofelectron-hole pairs at the time when high electric field is applied.

As the material for the amplification layer 25, a--SiC is used in thisembodiment. Usable examples further include Se, a--SiN, a--SiO, CdS andZnS and compounds thereof. It is to be noted upon selection thatincident light is irradiated from the side of the amplification layer 25in this embodiment so that the optical absorption coefficient of theamplification layer 25 should be made sufficiently smaller than that ofthe optical absorption layer 24.

As two a--SiC films forming the amplification layer 25, the band gapvalue (forbidden band width) Eg1 of the avalanche layer 25a is set at2.0 eV and that Eg2 of the barrier layer 25b is set at 3.5 eV.

Conditions for the formation of the a--SiC film are generally asfollows:

Gas used and flow rate: SiH₄ 100 sccm, C₂ H₆ several tens sccm

Pressure: 0.5 Torr

Substrate temperature: 250° C.

RF Power: 50 W

Film thickness: 10-1000 nm

Here, a desired band gap value can be obtained by changing the flow rateof a C₂ H₆ gas as needed and adjusting the C content. In thisembodiment, the gas flow rate for obtaining a film (avalanche layer 25a)of a forbidden band width Eg1 is set at 10 sccm and that for obtaining afilm (barrier layer 25b) of a forbidden band width Eg2 is set at 200sccm. The avalanche layer 25a and barrier layer 25b are each formed togive a film thickness of 10 nm and the barrier layers 25b are insertedbetween each contiguous two layers of the five avalanche layers 25a. Inaddition to the film thickness described above, the film thickness ofthe avalanche layer 25a which causes avalanche amplification can be 5 nmto 100 nm, while that of the barrier layer 25b can be 5 nm to 50 nm.

Next, the a--SiC lamination film which is the amplification layer 25 iscrystallized using a laser beam. Examples of the crystallization methodusable in this embodiment include, in addition to the above method, Arlaser, CO₂ laser and infrared flash annealing. It should however benoted that the crystallization method is selected so that heat rays arefully absorbed by the avalanche layer 25a to cause crystallization ofthe layer, and they are absorbed only by the avalanche layer 25a and donot exert influence on the barrier layer 25b and the optical absorptionlayer 24 therebelow. In this embodiment, there exists an a--Si layer(optical absorption layer 24) below the amplification layer. If heatrays reach the optical absorption layer and cause crystallization ofa--Si, the optical absorption layer cannot exhibit its desiredperformance, so the selection of the heat rays has an important meaning.

In this embodiment, a laser having a wavelength of 350 nm within a bluelight range is selected so that the absorption coefficient of theavalanche layer 25a (a--SiC of the forbidden band width Eg1) issufficiently large and the light almost transmits the barrier layer 25b(SiC of the forbidden width band Eg2). So, if the film thickness ofa--SiC and laser strength are set suitably, only the avalanche layer 25acan be crystallized and at the same time, the barrier layer 25b andoptical absorption layer 24 can be maintained as they are.

The object of the formation of the amplification layer as describedabove is to crystallize only the avalanche layer 25a of a forbidden bandwidth Eg1, said avalanche layer being requested to have high filmquality and being a layer which causes avalanche amplification, and inaddition, to maintain the interface between the avalanche layer 25a andthe barrier layer 25b steep. When electrons traveling in theamplification layer 25 transfer from the layer (barrier layer 25b) ofthe forbidden band width Eg2 to the layer (avalanche layer 25a) of theforbidden band width Eg1, they gain, at their interfacial part, energycorresponding to ΔEc between these two layers, thereby causing avalancheamplification. Accordingly, it is necessary to give energy to electronseffectively by suppressing the diffusion of C and Si atoms between thesetwo layers and forming a steep interface.

A typical example of the crystallizing conditions is shown below:

Laser used: wavelength 350 nm

Laser strength: 50-500 mW/cm²

Pulse width: 50 nsec

Pulse irradiation frequency: 1-50 times

Under the above conditions, only the avalanche layer 25a (a--SiC layerhaving Eg1 of 2.0 eV) is crystallized.

The gap level density of a--SiC (avalanche layer 25a) reduceddrastically from 10¹⁸ /cm³ to 10¹⁶ /cm³ by the crystallization accordingto the above method. For the crystallization of the avalanche layer 25awhich is thicker, a laser of a longer wavelength can be used or laserstrength can be increased.

It is also possible to conduct, as needed, various treatments for theimprovement of the film quality of a--SiC which is the amplificationlayer 25. Examples of the effective treatment include hydrogenationtreatment by the plasma CVD or ECR CVD apparatus and annealing in aforming gas (H₂, N₃). When the a--SiC layer is treated, hydrogenationtreatment using plasma is effective for making up for H (hydrogen) whichhas been once contained in the film but released during thecrystallization step, thereby reducing the gap level in the film.

On the amplification layer 25, a hole-injection inhibition layer 26 onwhich an n⁺ a--Si film has been deposited to the film thickness of 50 nmis formed. As the hole-injection inhibition layer 26, insulating filmssuch as SiO_(x) and Si_(x) N_(y) films are also usable.

On the hole-injection inhibition layer 26, formed is an upper electrode27 obtained by depositing ITO, SnO₂ or the like to form a transparentelectrode and then patterning. Any material can be used for theelectrode insofar as it has a sufficient transmittance to the light andhas low resistance. Examples include ultra thin metal coating (1-10 nm)such as Al, Cr, Ta, Ti and Ni and alloys thereof and also suicidesthereof.

In this embodiment, an example of incident irradiation of light from theupper part of the semiconductor photo diode (on the side of the upperelectrode 27) is shown, but it is also possible to irradiate the lightincident from the lower part of the semiconductor photo diode using theinsulating substrate 21 as a light transmitting material. At this time,the upper electrode 27 is not required to be transparent.

According to this embodiment, avalanche amplification is conducted inthe avalanche layer 25a which corresponds to the amplification layer 14of the first embodiment, so the similar effects to those of the firstembodiment can be obtained. In addition, avalanche amplification iscaused by the energy corresponding to ΔEc between the avalanche layer25a and the barrier layer 25b at the interfacial part therebetween,which makes it possible to bring about a further improvement in theamplification factor.

FIG. 7 illustrates one example of the characteristics of thesemiconductor photo detector fabricated by the above method in the casewhere the thickness of the optical absorption layer 24 (a--Si) is set at500 nm and that of the amplification layer 25 (a--SiC) is set at 100 nmin total. In this embodiment, the amplification layer 25 has amultilayer laminate structure composed of the avalanche layer 25a andthe barrier layer 25b, whereby a higher gain can be obtained comparedwith the properties (FIG. 4) of the semiconductor photo detector shownin the first embodiment.

A description will next be made of specific embodiments of thesemiconductor photo detector according to the present invention, whichcorresponds to the above-described structure (ii).

Embodiment 3

The semiconductor photo detector according to this embodiment isbasically equal to that shown in FIG. 8. Described specifically, on aninsulating substrate 1, a lower electrode 2, an amplification layer 3,an optical absorption layer 4 and an upper electrode 5 are stacked oneafter another.

First, on the insulating substrate 1 made of glass, ceramic or the like,a desired pattern is formed as the lower electrode 2 by using a metalsuch as Cr, Al, Ta, Ti, Mo or Ni. Further examples of the metal mayinclude alloys of the above-exemplified metal. Other materials havinggood conductivity can also be used.

On the lower electrode 2, a--Si is deposited, as the first layer formingthe amplification layer 3, to give a film thickness of 200 nm by usingthe plasma CVD method or LP CVD method, whereby a well layer 3a isformed. The well layer 31 may be deposited to the film thickness fallingwithin a range of from 10 nm to 1000 nm. The general conditions for thedeposition using LP CVD are as follows:

Gas used and flow rate: Si₂ H₆ (100%) 100 sccm

Pressure: 0.3 Torr

Substrate temperature: 480° C.

Examples of the deposition method usable here include, in addition tothe above method, ECR method, optical CVD method, sputtering method andthe vapor deposition method. In this embodiment, Si is used for the welllayer 3a of the amplification layer 3, but it is also possible to employSiGe or SiC to obtain a necessary band gap value.

In the above-described deposition, the Si film forming the well layer 3ais in the amorphous condition. Without crystallization, the intra-filmgap level (local level density) is 10¹⁷ /cm³ or higher. If voltage isapplied for amplification, a large amount of electron-hole pairs areformed from the above gap level, resulting in large dark current, whichcauses a marked lowering in the SN ratio and dynamic range of thesemiconductor photo detector.

As countermeasures against the above problem, it is very effective toirradiate laser light to the above-described Si film (amorphous film),thereby polycrystallizing the film. The polycrystallization to obtain apoly-Si film reduces the gap level of the Si film, whereby 10¹⁶ /cm³ orsmaller which is the gap level necessary for suppressing the darkcurrent can be obtained easily. As a result, not only the dark currentin the well layer 3a can be reduced remarkably but also the mobility ofelectrons in the film can be improved by two figures, which make itpossible to bring about a large improvement in the mean free path ofelectrons, thereby increasing the electric current and attaining animprovement in the amplification factor.

In the above embodiment, an excimer laser is employed upon theabove-described crystallization of the amorphous Si film. As one of themerits of using the excimer laser is that because it has a shortwavelength in a ultraviolet region, light is almost absorbed in the Sifilm and there is no thermal influence on the lower insulating substrate1 or the like. Crystallizing conditions in this embodiment are asfollows:

Laser used: KrF

Laser strength: 50-500 mW/cm²

Pulse width: 50 nsec

Pulse irradiation frequency: 1-50 times

Under these conditions, the well layer 3a (a--Si layer) was crystallizedto give a film thickness of about 200 nm.

Next, as the second layer of the amplification layer 3, the barrierlayer 3b is formed by depositing amorphous SiC through the plasma CVDmethod. Examples of the usable material include, in addition to SiC, SiOand SiN. Examples of the formation method include, in addition to theplasma CVD, ECR CVD, optical CVD, sputtering and vapor depositionmethod.

Upon film deposition of amorphous SiC, the band gap of the barrier layer3b is formed so as to show a continuous and gradual decrease from theside of the well layer 3a by changing the composition ratio of Si and C.In other words, in the barrier layer 3b, the band gap is formed to showa continuous increase from the side of the optical absorption layer 4 tothe side of the well layer 3a. The band gap at the interfacial part onthe side of the well layer 3a is formed so that the value of thedifference (ΔEc) of the electron conduction band at the interfacial partbetween the well layer 3a and the barrier layer 3b becomes larger thanthe value of the band gap (forbidden band width Eg) of the materialforming the well layer 31.

Owing to such structure, the electrons which have transferred from thebarrier layer 3b to the well layer 3a receive the energy correspondingto the difference of the conduction band at the interfacial part(convert the potential energy to the kinetic energy) and with only thisenergy, avalanche amplification is caused in the well layer 3a, whichmakes it possible to cause a high-sensitivity amplification operation ofthe semiconductor photo detector.

The conventional APD which accelerates electrons by externally appliedvoltage, thereby causing an avalanche phenomenon, while according to theenergy transfer structure of the above-described semiconductor photodetector, application of external electric field to cause an avalanchephenomenon is not required [reverse bias voltage is necessary forflattening or monotonously decreasing the energy value of the conductionband of photo carriers in the barrier layer 3b (FIG. 8(c)].

Furthermore, a polycrystalline material is used as the well layer 3a sothat when reverse bias voltage is applied to the semiconductor photodevice, the barrier layer 3b becomes a high resistance layer relative tothe well layer 3a and the electric field is therefore hardly applied tothe well layer 3a, whereby unnecessary electron-hole pairs are notformed at all and the generation of the dark current is suppressed. As aresult, owing to the high mobility of the polycrystalline material,sensitivity increase can be attained.

In this embodiment, the well layer 3a is made of a poly-Si film so thatthe band gap (forbidden band width Eg) of the well layer is about 1.1eV. It is important that the value of the difference (ΔEc) of theconduction band exceeds it. Accordingly, amorphous SiC is used as amaterial for the barrier layer 3b on the side of the well layer 3a sothat the band gap value (forbidden band width Eg) becomes about 3.5 eVand the value of the difference (ΔEc) of the conduction band is designedto be about 1.6 eV.

It is desired that the band gap value at the interface of the barrierlayer 3b on the side of the optical absorption layer 4 is equal to orsmaller than that of the optical absorption layer 4. This is because theloss of electrons caused by the energy barrier at this part can beprevented at the time when the electrons transfer from the opticalabsorption layer 4 to the barrier layer 3b. In this embodiment,amorphous Si is used as the optical absorption layer 4, which will bedescribed later, so that the band gap value in this layer becomes about1.7 eV. So, the band gap value of the barrier layer 3b is allowed tochange successively from 3.5 eV on the side of the well layer 3a to 1.7eV on the side of the optical absorption layer 4.

Specific conditions for the formation of the barrier layer 3b accordingto this embodiment are as follows:

Gas used and the flow rate: SiH₄ 10-100 sccm C₂ H₆ 0-200 sccm

Pressure: 0.5 Torr

Substrate temperature: 250° C.

RF Power: 50 W

Film thickness: 10-1000 nm.

The C content is adjusted by changing the flow rate of a C₂ H₆ gas asneeded. In this way, during film deposition, the gas flow rate ischanged continuously.

After the formation of the amplification layer 3 composed of the welllayer 3a and the barrier layer 3b, a--Si is deposited as an opticalabsorption material, whereby the optical absorption layer 4 is formed.In this embodiment, as the optical absorption material, a--Si isemployed, but it is also possible to add a suitable additive to Sidepending on the wavelength of the light to be detected. For example, inorder to increase the sensitivity to the shorter wavelength region, adesired amount of C, N, O or the like is added, while for increasing thesensitivity to the longer wavelength region, an element such as Ge isadded.

As the base material for the optical absorption material, it is desiredto use Si in the case where a visible light range is detected, but otherthin film materials such as Ge, Se, CdS, CdSe or PbS can also beemployed.

As the final step, ITO, SnO₂ or the like which is a light transmittingmaterial is deposited for film formation, followed by patterning wherebythe upper electrode 5 is formed as a transparent electrode. Anyelectrode material can be used insofar as it has both a sufficient lighttransmittance and small resistance. Examples include ultra thin films(1-10 nm) of metals such as Al, Cr, Ta, Ti and Ni and alloys thereof.Silicides thereof can also be used.

As illustrated in FIG. 10, between the optical absorption layer 4 andthe transparent electrode 5 and the amplification layer 3 and theelectrode 2, a p layer 6 and an n layer 7 each made of amorphous orpolycrystalline Si are inserted, as needed, as an electron blockinglayer and a hole blocking layer, respectively, whereby effects forreducing the dark current can be obtained. Instead of these p layer 6and n layer 7, thin insulation films of 1-100 nm can be inserted tobring about similar effects.

In this embodiment, incident light is irradiated from the upper part ofthe semiconductor photo detector, but it is possible to form theinsulating substrate 1 using a light transmitting material and toirradiate the incident light from the lower part of the semiconductorphoto detector. In this case, it is not necessary to form the upperelectrode 5 from a light transmitting material.

Using the above-described method, a semiconductor photo detector whichhas a film thickness of 500 nm in the optical absorption layer 4 (a--Sifilm), and 200 nm each in the well layer 3a and the barrier layer 3b ofthe amplification layer 3 was fabricated and its characteristics weremeasured. As a result, it has about 50 gains at the applied voltage of10V, showing an improvement in the amplification factor by at least onefigure compared with the conventional embodiment. It is presumed that afurther improvement can be obtained by increasing the film thickness andapplied voltage. The dark current at the applied voltage of 20 V is 1nA/cm² or lower and it has been confirmed that the dark current loweredby two figures or more.

Embodiment 4

A description will next be made of a semiconductor photo detectoraccording to the fourth embodiment of the invention, with reference toFIG. 11. FIG. 11(a) illustrates a basic constitution of thesemiconductor photo detector which is similar to that of Embodiment 3except for the amplification layer. FIG. 11(b) is a schematic viewillustrating an energy band structure of the semiconductor photodetector at the time when no voltage is applied.

With a view to bringing about further improvement in the gain in theconstitution shown in Embodiment 3, the amplification layer 3 accordingto this embodiment is designed to have a multilayer laminate structureof super lattice obtained by stacking plural pairs, each pair formed ofa well layer 3a and a barrier layer 3b. As described in Embodiment 3,from the difference in the energy of the conduction band (band offset)at the interfacial part between the well layer 3a and the barrier layer3b, which form the amplification layer as a pair, electrons receiveenergy and cause an avalanche amplification phenomenon in the well layer3a. Accordingly, it becomes possible to heighten the sensitivity furtherby changing the basic structure to the multilayer lamination.

The multilayer laminate structure is formed by stacking theamplification layer 3 composed of five layers, each layer being formedof the well layer 3a and the barrier layer 3b, which is shown inEmbodiment 3. The well layer 3a and the barrier layer 3b are depositedcontinuously by using the plasma CVD. Although it is also possible toform the well layer 3a and the barrier layer 3b by the differentmethods, it is desired to conduct continuous formation by the samemethod as in this embodiment, because impurities tend to be mixed in theinterface between these two layers, if different methods are employed.

Conditions for the film deposition of the well layer 3a by the plasmaCVD are as follows:

Gas used and flow rate: SiH₄ (100%) 100 sccm

Pressure: 0.3 Torr

Substrate temperature: 250° C.

RF Power: 50 W

Film thickness: 20 nm

On the well layer 3a, an SiC layer is formed to give a film thickness of100 nm while a band gap is changed continuously under the conditionssimilar to those for the barrier layer 3b described in Embodiment 3,whereby the amplification layer 3 is formed. Then five amplificationlayers 3 each composed of the well layer 3a and the barrier layer 3b aredeposited continuously, whereby a multilayer laminate structure isobtained.

This multilayer laminate structure is then crystallized to effect thesensitivity increase and the dark current reduction as shown inEmbodiment 3. It should be noted that different from Embodiment 3,crystallized is a laminate structure and because of the continuouslamination, the Si film of the well layer 3a is formed of an amorphousfilm containing hydrogen.

In this embodiment, the film thickness of the amplification layer havinga multilayer laminate structure is about 1 μm in total so that when alaser of short-wavelength light is used, as in the crystallizationmethod shown in Embodiment 3, optical absorption occurs only in thesurface layer portion of the film and crystallization of the wholelaminated film cannot be conducted. Examples of the method forcrystallization include the lamp annealing method and furnace annealingmethod in which crystallization is conducted using the light of a longerwavelength such as blue-color laser or infrared laser and conducted in ahigh-temperature atmosphere, respectively. In this embodiment, flashlamp annealing using heat rays is employed.

As described above, a large amount of hydrogen is contained in the Sifilm of the well layer 3a. If crystallization is conducted by directlyexposure to high energy, bumping of intra-film hydrogen occurs,resulting in the formation of many openings in the film. To avoid this,pre-annealing is conducted at low energy for dehydrogenation prior tothe crystallization of the well layer 3a.

Pre-annealing conditions for the dehydrogenation are as follows:

Lamp power: 100 W (gradual heating)

Irradiation time: 1 sec

Irradiation frequency: once.

Annealing conditions subsequent to the pre-annealing for dehydrogenationare as follows:

Lamp power: 300 W

Irradiation time: 0.1 sec

Irradiation frequency: 5-10 times.

Under the above conditions, the crystallization for the amplificationlayer (having a film thickness of about 1 μm) having a multilayerlaminate structure was conducted.

The other layers were formed as shown in Embodiment 3.

Embodiment 5

In the semiconductor photo detector according to Embodiment 5, thestructure and the formation method of each layer are similar to those ofEmbodiment 4 except for the crystallization step of the well layer 3a.In Embodiment 4, blue laser light is employed as a crystallizationmeans. According to the crystallization step shown in Embodiment 4, thewhole multilayer laminate structure is molten and crystallized so thatdiffusion of a small amount of an element (C in this case) contained asan additive in the barrier layer 3b occurs at the interfacial partbetween the well layer 3a and the barrier layer 3b and the band offsetpart at the interfacial part between the well layer 3a and the barrierlayer 3b has acquired a somewhat gently-sloping structure (the structureof the energy band showing a continuous change) as shown in FIG. 9.

In the actual operation, the difference (ΔEc) of the conduction band atthe interface between the well layer 3a and the barrier layer 3b issufficiently large relative to the band gap (forbidden band width Eg) ofthe well layer 3a so that an avalanche phenomenon can be caused only bythis energy without any problem, and as another merit, as describedabove, a surface level which otherwise becomes a recombination centeralmost disappears. When electrons which have transferred from thebarrier layer 3b to the well layer 3a receive, at the interfacial parttherebetween, the energy corresponding to the difference of theconduction band (potential energy is converted to the kinetic energy),it is preferred to steeply change the energy band at the interfacialpart to suppress the loss of the energy at this time to the minimum.

So, in Embodiment 5, with a view to suppressing the loss to the minimumand to cause a steep change, a blue laser light having a wavelength of350 nm was used as a light source for crystallization. According to thecrystallization step by the blue laser light, light is absorbed by thematerial of a narrow band gap which forms the well layer 3a but ittransmits through the interfacial part which exists between the welllayer 3a and the barrier layer 3b and has a large band gap so that onlythe well layer 3a can be crystallized. It is therefore possible tosuppress the diffusion of C from the interfacial part on the side of thebarrier layer 3b and form a steep interface.

Embodiment 6

In the semiconductor photo detector according to Embodiment 6, thestructure and the formation method of each layer are similar to those ofEmbodiment 4 except for the film thickness of the whole multilayerlaminate structure. Described specifically, the well layer 3a and thebarrier layer 3b are each formed to the film thickness of 20 nm and thewhole multilayer laminate structure is formed to the thickness of 200nm. It is preferred to set the film thickness of the amplification layer3 composed of the well layer 3a and the barrier layer 3b as thick aspossible to cause an avalanche phenomenon of electrons by the electricfield. The high amplification factor can however be obtained even whenthe semiconductor photo detector has the above film thickness (the filmthickness of the whole multilayer laminate structure: 200 nm).

In each Embodiment 4 and Embodiment 5, the total film thickness of themultilayer laminate structure was as thick as about 1 μm socrystallization means using ultraviolet light could not be employed. Inthis Embodiment, however, crystallization of the well layer 3a wasconducted using ultraviolet rays, similar to Embodiment 3. Conditions inthe crystallization step are similar to those shown in Embodiment 3.

For the thin multilayer laminate structure as shown in this Embodiment,crystallization means using lamp annealing or blue laser as described inEmbodiments 4 and 5 can be employed.

What is claimed is:
 1. A process for making a semiconductor photodetector comprised of a lower electrode and an upper electrode, whereinat least one of the lower electrode and the upper electrode has a lighttransmitting property, and between the lower electrode and the upperelectrode, an optical absorption layer which generates photocarriers,receiving light and an amplification layer which amplifies thephotocarriers formed by the optical absorption layer, the processcomprising:stacking the lower electrode, the optical absorption layer,the amplification layer and the upper electrode, wherein at least theamplification layer is stacked in an amorphous condition, andcrystallizing the amplification layer after stacking the amplificationlayer.
 2. A process for making a semiconductor photo detector accordingto claim 1, wherein the crystallizing comprises bringing anamplification layer into a high temperature atmosphere until theamplification layer is crystallized.
 3. A process for making asemiconductor photo detector according to claim 1, wherein thecrystallizing is conducted by infrared-ray lamp annealing or furnaceannealing.
 4. A process for making a semiconductor photo detectoraccording to claim 1, wherein the crystallizing comprises exposing theamplification layer to light of a wavelength not absorbable by theoptical absorption layer but absorbable by the amplification layer.
 5. Aprocess for making a semiconductor photo detector according to claim 1,wherein during the crystallizing, the process further comprises meltingan interfacial region of the optical absorption layer and theamplification layer.
 6. A process for making a semiconductor photodetector comprised of a lower electrode and an upper electrode, whereinat least one of the lower electrode and the upper electrode has a lighttransmitting property, and between the lower electrode and the upperelectrode, an optical absorption layer which generates photocarriers,receiving light and an amplification layer which amplifies thephotocarriers formed by the optical absorption layer, wherein theamplification layer comprises a barrier layer having band gap largerthan that of the optical absorption layer and a well layer stackedcontiguously on the barrier layer, the process comprising:stacking thelower electrode, the optical absorption layer, the barrier layer, thewell layer, and the upper electrode, wherein at least the well layer isstacked in an amorphous condition; and crystallizing the well layerafter stacking the well layer.
 7. A process for making a semiconductorphoto detector according to claim 6, wherein the crystallizing comprisesbringing the well layer into a high temperature atmosphere until thewell layer is crystallized.
 8. A process for making a semiconductorphoto detector according to claim 6, wherein the barrier layer is alsostacked in an amorphous condition, and the crystallizing alsocrystallizes the barrier layer.
 9. A process for making a semiconductorphoto detector according to claim 6, wherein the barrier layer is alsostacked in an amorphous condition, and the crystallizing comprisesexposing the barrier layer and well layer to light of a wavelengthabsorbable by the barrier layer and well layer after stacking at leastthe barrier layer and well layer to crystallize the barrier layer andwell layer.
 10. A process for making a semiconductor photo detectoraccording to claim 6, wherein the barrier is also stacked in anamorphous condition, and the crystallizing comprises exposing thebarrier layer and well layer to light of a wavelength not absorbable bythe barrier layer but absorbable by the well layer after stacking atleast the barrier layer and the well layer, thereby crystallizing thewell layer.
 11. A process for making a semiconductor photo detectoraccording to claim 10, further comprising a dehydrogenation step forforming at least the well layer from hydrogenated Si in the stackingstep and subsequently dehydrogenating the well layer prior to thecrystallizing step.
 12. A process for making a semiconductor photodetector according to claim 6, wherein the well layer is a crystalsubstance following the crystallizing such that, at the interface withthe barrier layer, an energy value of the conduction band of thephotocarriers is lower than that in the barrier layer, and a differencein the energy value of the conduction band of the photocarriers betweenthe well layer and the barrier layer is larger than the bandgap betweenthe valence band and conduction band of the well layer.